Wednesday, August 03, 2016

Artist’s concept of the atmospheric collapse of Jupiter’s volcanic moon
Io, which is eclipsed by Jupiter for two hours of each day (1.7 Earth
days). The resulting temperature drop freezes sulfur dioxide gas,
causing the atmosphere to “deflate,” as seen in the shadowed area on the
left. Credits: SwRI/Andrew Blanchard. Full resolutionJPEG

Gemini observations show that the thin atmosphere of Jupiter's moon Io
undergoes dramatic changes during frequent eclipses with the giant
planet. The following press release, issued by the Southwest Research
Institute, explains how the dramatic changes in temperature cause the
moon's atmosphere to collapse.

San Antonio – Aug. 2, 2016 – A Southwest Research Institute-led team
has documented atmospheric changes on Io, Jupiter’s volcanically active
satellite, as the giant planet casts its shadow over the moon’s surface
during daily eclipses.

A study led by SwRI’s Constantine Tsang concluded that Io’s thin atmosphere, which consists primarily of sulfur dioxide (SO2) gas emitted from volcanoes, collapses as the SO2
freezes onto the surface as ice when Io is shaded by Jupiter. When the
moon moves out of eclipse and ice warms, the atmosphere reforms through
sublimation, where ice converts directly to gas.

“This research is the first time scientists have observed this
phenomenon directly, improving our understanding of this geologically
active moon,” said Tsang, a senior research scientist in SwRI’s Space
Science and Engineering Division.

The findings were published in a study titled “The Collapse of Io’s
Primary Atmosphere in Jupiter Eclipse” in the Journal of Geophysical
Research. The team used the eight-meter Gemini North telescope in
Hawai'i with the Texas Echelon Cross Echelle Spectrograph (TEXES) for
this research.

Data showed that Io’s atmosphere begins to “deflate” when the
temperatures drop from -235 degrees Fahrenheit (-148 ℃) in sunlight to
-270 degrees Fahrenheit (-168 ℃) during eclipse. Eclipse occurs 2 hours
of every Io day (1.7 Earth days). In full eclipse, the atmosphere
effectively collapses as most of the SO2 gas settles as frost
on the moon’s surface. The atmosphere redevelops as the surface warms
once the moon returns to full sunlight.

“This confirms that Io’s atmosphere is in a constant state of collapse
and repair, and shows that a large fraction of the atmosphere is
supported by sublimation of SO2 ice,” said John Spencer, an
SwRI scientist who also participated in the study. “Though Io’s
hyperactive volcanoes are the ultimate source of the SO2,
sunlight controls the atmospheric pressure on a daily basis by
controlling the temperature of the ice on the surface. We’ve long
suspected this, but can finally watch it happen.”

Prior to the study, no direct observations of Io’s atmosphere in eclipse
had been possible because Io’s atmosphere is difficult to observe in
the darkness of Jupiter’s shadow. This breakthrough was possible because
TEXES measures the atmosphere using heat radiation, not sunlight, and
the giant Gemini telescope can sense the faint heat signature of Io’s
collapsing atmosphere.

Tsang and Spencer’s observations occurred over two nights in November
2013, when Io was more than 420 million miles (676 million km) from
Earth. On both occasions, Io was observed moving in and out of Jupiter’s
shadow, for a period about 40 minutes before and after eclipse.

Io is the most volcanically active object in the solar system. Tidal
heating, the result of Io’s gravitational interaction with Jupiter,
drives the moon’s volcanic activity. Io’s volcanoes emit umbrella-like
plumes of SO2 gas extending up to 300 miles (483 km) above
the moon’s surface and produce extensive basaltic lava fields that can
flow for hundreds of miles.

This study is also timely given that NASA’s Juno spacecraft entered
Jupiter orbit on July 4th. “Io spews out gases that eventually fill the
Jupiter system, ultimately seeding some of the auroral features seen at
Jupiter’s poles,” Tsang said. “Understanding how these emissions from Io
are controlled will help paint a better picture of the Jupiter system.”